Effect of pH on Surface Texture Of Glass Ionomer Containing /based Restorative Materials: An In-vitro Study

INTRODUCTION

Glass ionomer cements have gained popularity due to their favourable adhesive and fluoride-releasing properties. However, they exhibit few disadvantages which include sensitivity to moisture, low initial mechanical properties and inferior translucency compared to resin composites.

To overcome these disadvantages hybrid materials combining the technologies of glass ionomers and resin composite were subsequently developed which include resin-modified glass ionomer cements, compomers (polyacid-modified resin composites) and a new category of hybrid aesthetic restorative material, Giomers, which employ the use of pre-reacted glass ionomer (PRG) technology to form a stable phase of glass ionomer. The fluoroalumino silicate glass in these materials is reacted with polyalkenoic acid in water prior to inclusion into the silica-filled urethane resin. This technology differs from compomers, in which a variable amount of dehydrated polyalkenoic acid is incorporated into the resin matrix. The acid does not react with the glass until water uptake into the restoration occurs. Giomers come in a one-paste form and manufacturers' claims include fluoride release and recharge, biocompatibility, clinical stability, excellent aesthetics and smooth surface finish² .

Dental erosion is defined as the irreversible loss of dental hard tissue by a chemical process without the involvement of micro-organisms⁵ and is due to either extrinsic or intrinsic acid sources. Intrinsic acids are produced by recurrent vomiting in  patients with anorexia, bulimia or gastro oesophageal reflux⁶ and extrinsic acids are derived from the environment, medications, lifestyle and diet⁷ . The over consumption of dietary acids in the form of soft drinks has been linked to tooth surface loss⁸ . It was reported that in the year 2002, Americans consumed approximately 53 gallons of soft drinks per person per year and there is an increasing trend to consumption of fruit drinks in children^9.

Exposure to either extrinsic or intrinsic acids may adversely affect dental restorations, as the low pH level may cause erosion of the glass-ionomer based/containing materials and leaching of the principle matrix forming substances^{10,11,12,13,14}. Studies that looked at the effects of low pH drinks on the properties of restorative materials showed that composite, resin-modified (RM-GIC) and conventional glass ionomer cements (GIC) were more resistant to varying pH levels than dental enamel¹⁵ . However, acid resistance of conventional GIC was consistently less than RM-GIC and composite.

The surface roughness of direct tooth colour restorative materials is significantly affected by conditioning in different food simulating liquids, including acids^16,11 . The surface texture of tooth-colour restorative materials affects the lifespan of the restorations. The presence of irregularities on the surface of materials may influence appearance, plaque retention, surface discoloration and gingival irritation¹⁷ . The increasing use of glass-ionomer based/containing restorative materials and the recent trend towards increased consumption of acidic drinks necessitates studying the effects of pH level on these restorative materials. This study determined the effect of pH on the surface texture of glassionomer based/containing restorative materials used for the restorations of posterior teeth.

METHODOLOGY

A compomer (Dyract), a giomer (Beautifil II) and two highly viscous glass ionomer cements (Fuji IX and KetacMolar) were used in this study with a resin composite (Spectrum TPH) as a control. The materials were placed in the cylindrical recesses (5-mm diameter x 2mm deep) of customized acrylic moulds and covered with Mylar strips. A glass slide was placed over the acetate strip and pressure was applied to extrude the excess material. The light-cured restorative materials were then light-polymerized according to the manufacturers' instructions through the glass slide using a curing light 2500 (3M-ESPE, St Paul, MN, USA).

The two highly viscous glass ionomer cements were activated / mixed according to the manufacturers' directions. Immediately after setting, the acetate strips were discarded and the specimens stored in deionized distilled water for two weeks at 37°c.

Thirty six specimens were made for each restorative material. They were randomly divided into six groups of six specimens and conditioned in citric acid solutions of differing pH (pH 2, 3, 4, 5 and 6) levels at 37°C for one week. The pH of the citric acid solutions was adjusted by using citrate buffers. A stock solution of citric acid was prepared by using 496.1 gems of pure citric acid dihydrite and 298.2 gems of sodium citrate, mixed in distilled water with constant stirring until a uniform solution was formed. To the stock solution thus obtained citrate was added to decrease the pH and sodium hydroxide was added to increase the pH. The solution was thus adjusted to pH 2,3,4,5, and 6. Deionized distilled water was used for pH 7.

The pH of the solutions was measured using a pH meter (LI 612 pH Analyser, ELICO). After the one-week conditioning period, the samples were lightly rinsed with water and dried. Surface roughness was then determined using a Profilometer (WYKO Surface Profilers; Veeco Metrology Group, Arizona, USA) with a probe diameter of 5µm. Ra values for each specimen were taken across the centre of each specimen over a standard length of 0.25mm X 4. Ra value is the arithmetic mean of the departures of the roughness profile from the mean line calculated by the computer. All statistical analysis was carried out at significance level 0.05. Two-way ANOVA was used to determine material-pH interactions. One-way ANOVA and Scheffe's post-hoc tests were used to compare the surface roughness of the materials after conditioning in acids of different pH.

RESULTS

Table I and graph 1 & 2 show the mean Ra value for different materials at different pH levels. The result of statistical analysis are reflected in table

The effects of pH on surface roughness were material dependant. Ra values ranged from 0.05 to 0.34µm and 0.07 to 4.42µm for pH 7 and pH 2 respectively. With the exception of Spectrum TPH, significant differences in values were observed between pH. For Dyract specimens conditioned at pH 2, 3, 4 & 5 were rougher than those conditioned at 6&7. For Beautifil – II, Fuji IX and KetacMolar conditioned at pH 2,3&4 were significantly rougher than those conditioned at pH 5&6.

The materials exhibited different degrees of surface degradation at different pH levels. At pH 3 Fuji IX and KetacMolar were significantly rougher than the other materials. KetacMolar was roughest material at pH2,4,5,6&7. Fuji IX was roughest material at Ph.

DISCUSSION

The aesthetics and longevity of tooth colour restoratives are highly dependent on their surface characteristics. Residual surface roughness of restoration encourages plaque accumulation, which may result in gingival inflammation, superficial staining and secondary caries¹⁶ . The materials evaluated in this study represent the entire continuum of direct aesthetic restorative materials currently available to the dental practitioner.

The method of obtaining different pH of the citric acid solution has been described in earlier. The other method of obtaining different pH of the citric acid solution by adding distilled water that was also used as pH 7. But the latter method is volume and temperature dependent, any change in either the volume of the water or the temperature would change the pH value of the solution. On the contrary the solution prepared from the former method was very stable irrespective of change in the temperature or volume.

The effect of pH on surface roughness was material dependent and only the composite material (Spectrum TPH) and Dyract were not significantly affected by varying pH levels. The compomer (Dyract) demonstrated significantly lower surface roughness than other glass-ionomer based restorative materials (Fuji IX and Ketac Molar) after conditioning in the acids but the roughness was not statistically significantly different from that of Spectrum TPH at all pH levels. In fact, the surface roughness of Dyract has been reported to more closely approximate resin composites¹⁸ . The Giomer (Beautifil) exhibited surface degradation at levels between the Compomer and the highly viscous GIC when conditioned at varying pH levels. It was found that the surface roughness of Beautifil was not significantly different from Spectrum or Dyract at all pH except pH 2 and 3 . For all materials tested, the smoothest surface texture was found when they were conditioned in deionized distilled water a neutral pH 7, even though the mean Ra values differed between the materials, with Fuji IX and Ketac Molar being significantly rougher than the rest. All the materials tested can be considered biphasic, with one phase embedded in the other. Glass ionomers consist of glass particles in a hydrogel matrix and composites consist of glass filler particles in a polymer resin² . In acidic solutions, H⁺ ions of citric acid diffused into the glass ionomer components and replaced metal cations in the matrix. These free cations would diffuse outward and be released from the surface. As the metal cations in the matrix decreased, more would be extracted from the surrounding glass particles, causing them to dissolve¹⁷ . Consequently, the material would present a rough surface with voids and protruded, undissolved glass particles. Prolonged exposure of these glass ionomer based/containing materials to acids would result in higher Ra values recorded by the profilometer. The hydrogen ions get readily diffused through the hydro gel matrix of the glass ionomers than through the cured polymer resins. Solubility of dental resin based composite materials, depend on several factors including, chemistry of the monomer resins, the extent of polymerization of the polymer matrix, filler particle size, shape, and distribution and the interfacial properties between the filler and resin matrix (Kovarik et al, 2005). Furthermore other factors like the number of fluoride ions released being greater, the particle size of filler particles being greater than the composite resins and in addition the minute air porosities included during mixing of glass ionomers attribute to the increased surface roughness of glass ionomer cements than the composite resins.

It is believed that a threshold surface roughness for bacterial retention (Ra = 0.2 µm) exists below which no further reduction in bacterial accumulation can be expected. An increase in surface roughness above this threshold roughness, however, will result in a simultaneous increase in plaque accumulation, thereby, increasing the risk for both caries and periodontal inflammation¹⁵ . The materials with Ra values above the 0.2 µm threshold are Fuji IX (pH 2 and 3) and Ketac Molar (pH 2, 3 and 4) and Beautifil II (pH2) when conditioned in acids. This deterioration in the surface texture is more likely to cause increased bacterial adhesion, besides causing a clinically rough and dull surface. Therefore, careful selection of posterior restorative material is important in patients with a history of gastric reflux, eating disorders or high consumption of dietary acids. From the results of this study, it is hypothesized that there is a critical pH unique to individual materials at which the surface texture was significantly affected. This requires further investigation.

CONCLUSIONS

1. The effects of pH on the surface texture of glassionomer based/containing restoratives are material dependent. 
2. With the exception of composites, the surface roughness of all materials evaluated was significantly affected by acids of low pH.
3. The surface texture of highly viscous glass ionomer cements deteriorated significantly when conditioned in solutions of low pH, which makes them susceptible to bacterial adhesion.